The document EP-A-0 770 514 describes an example of such a hinge mechanism, which can stay unlocked even after releasing the control part when the seat back is tipped forward, for example in order to access rear places in the vehicle if the mechanism in question is assembled on a front seat. When the back is then set upright again, the mechanism automatically re-locks itself as soon as the first peg comes opposite the corresponding break of the circular guide.
The mechanism described in this document gives complete satisfaction, but has however the disadvantage of always re-locking in the same angular position of the back when this back is set upright again after having tipped it forward, and this regardless of the previous adjustment of the aforesaid back.
The user of the seat must therefore again adjust the inclination of his back each time he sets it upright again after having tipped it forward.
Furthermore, the document EP-A-0 694 434 describes a hinge mechanism with toothed wedges which allows tipping the seat back forward and which automatically re-locks in the previously adjusted position of the back: this mechanism operates perfectly, but the toothed wedges of this mechanism, which allow the adjustment of the back inclination, mesh with a toothed segment which is itself movable relative to the second end plate and which is interlocked with this second end plate by means of other toothed wedges.
In this case, the two end plates are therefore connected to each other by means of two stages of toothed wedges, which tends to make the mechanism heavier and more complex.